OpenZFS 8435 - zpool.1m and zfs.1m: minor cleanup

OpenZFS 8435 - zpool.1m and zfs.1m: minor cleanup

3796 listsnapshots not documented in zpool man page

Authored by: George Melikov <mail@gmelikov.ru>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Yuri Pankov <yuripv@gmx.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Ported-by: George Melikov mail@gmelikov.ru
OpenZFS-issue: https://www.illumos.org/issues/8435
OpenZFS-commit: openzfs/openzfs@a058d1c

Porting notes: OpenZFS review applied,
some ZoL changes were reverted.
See https://github.com/openzfs/openzfs/pull/415